1. Field of the Invention
This invention relates generally to remediation of toxic organic compounds in the aqueous phase. More specifically, this invention relates to processes and reaction vessels that use simultaneously ultraviolet light (UV) and ultrasonic waves (US) to destroy organic pollutants, typically halogenated organic compounds in water.
2. Related Art
Widespread water contamination with, for example, chlorinated volatile organic compounds (VOCs) has motivated research on photosonocatalysis technology for destroying VOCs, as an alternative to currently available remediation technologies such as air stripping and carbon adsorption processes.
Researchers have studied the effects of simultaneous and combined treatment of materials with ultraviolet light (xe2x80x9cphotolysisxe2x80x9d) and ultrasonic waves (xe2x80x9csonolysisxe2x80x9d).
Photolysis is a photochemical cleavage reaction initiated by the action of radiant energy in the UV/visible region of the electromagnetic spectrum. When light hits the molecule and the molecule absorbs its energy, the energy content of the molecule is increased and the molecule reaches an excited electron state. Photolysis is one deactivation process in which the unstable excited state molecule comes down to a stable ground-level energy state.
Photolysis occurs competitively with the radiation and radiationless reactions. In the former reaction, the molecules""s extra energy is released through emission of light (e.g., fluorescence). In the latter reaction, its extra energy is converted to thermal energy and dissipated. The absorption of light energy by the molecule may occur directly or indirectly. When the molecule itself directly adsorbs light energy and is degraded, the reaction is direct photolysis, while light energy is transferred from other substances, the resulting reaction is indirect or sensitized photolysis. When photons of light strike a semiconductor, they are either absorbed or scattered. The absorbed photons with energy greater than or equal to the semiconductor band gap energy excite electrons from the valence band to the conduction band. The excitation generates electron-hole pairs (exe2x88x92+h+) on semiconductors, which can either recombine and release heat, or cause oxidation and reduction reactions by charge transfer to species adsorbed to the semiconductor (Lau, 1996).
Sonolysis is a physical/chemical reaction initiated by implosion of cavitation bubbles in liquid, induced by ultrasound. Ultrasound can create powerful rarefaction waves to develop a negative pressure in liquid. If the waves are powerful enough to overcome the intermolecular forces of bonds in liquid, the liquid molecules will be torn apart from each other to form microbubbles in liquid. The cavitation bubbles are formed at the weak spots in the liquid. Once a microbubble is formed, it rapidly grows until it reaches the critical size at which the bubble can no longer sustain itself and results in an implosion instantaneously releasing a large amount of energy (Bhatnagar and Cheung, 1994).
The energy generated by the compression of gas and vapor inside is released as intense heat at a local hot spot. Suslick (1990) reported that these hot spots (imploding bubbles) would reach temperature of 5000xc2x0 C., pressure of 500 atmospheres, and heating and cooling rates greater than 109 K/sec. Thus, the reaction that takes place in aqueous solution may be direct bond cleavage, or thermal reaction similar to combustion. The extreme conditions may also produce reactive species (e.g., H2O2., HO2.H, .OH) in aqueous solution, and result in hydrolysis and oxidation-reduction reactions with these species.
Photosonolysis is the use of photolysis and sonolysis in combination. Initial studies on the use of photosonolysis were reported by Toy and Stringham (1984, 1985). They used ultraviolet light (UV) and ultrasonic waves (US) for the synthesis of 1,2,4-tris (methylthio)-3-H-hexafluoro-n-butane from methyl disulfide and hexafluorobutadiene. Photosonolysis was later applied in numerous additional synthetic applications, and has also been applied to polymer degradations (Toy and Stringham, 1985; Toy et al., 1990).
Toy and Stringham""s photosonosynthesis work has been expanded by other researchers to the applications to waste remediation technologies. Sierka and Amy (1985) studied the composite effects of ultraviolet light (UV), ultrasound (US), and ozone oxidation (O3) to reduce the trihalomethane formation potential and maximize the destruction of nonvolatile total organic carbons. They found that concurrent use of UV, US, and ozone provided the most effective combination in the performed experiments.
Johnston and Hocking (1991, 1993) studied the combined use of UV (with 0.5-2 g/L TiO2) and ultrasonic irradiation to degrade various chlorinated organics including pentachlorophenol (PCP, 2.4xc3x9710xe2x88x924 M); 3-chlorobiphenyl (PCB, 4xc3x9710xe2x88x924 M); 2,4-dichlorophenol (2,4-DCP, 1xc3x9710xe2x88x923 M); and 4-chlorophenol (4-CP). The chlorinated compounds were more rapidly degraded by the combination of ultrasound and ultraviolet light irradiation than with any other combination (i.e., UV or US treatment with or without TiO2) studied.
Toy et al. (1990) studied photosonocatalysis on decomposition of ethylene glycol and urea in aqueous solutions, and reported that photosonocatalytic decomposition is more aggressive than with either sonolysis or photolysis individually. Toy et al. (1990) also performed experiments on the photosonochemical decomposition of aqueous 1,1,1-trichloroethane (TCA), and reported that the combined use of photolysis and sonolysis causes a greater decomposition than if each technique were used separately.
Muzzoli et al. (1994) studied the effects of ultrasound and ultraviolet radiation on vitamin E and its pharmacological excipient, olive oil. The vitamin E appeared to be inactivated and behaved as a radical species, while olive oil appears unaffected by treatment either with ultrasound or with ultraviolet radiation.
Other inventors have developed methods and reactors for carrying out these treatments on liquids. See, for example, U.S. Pat. No. 3,672,823 (Boucher, issued Jun. 27, 1972), and U.S. Pat. No. 5,130,031 (Johnston, issued Jul. 14, 1992).
Still, there is a need for an easily constructed, simply operated, portable UV/US reactor design. Also, there is still a need for such a UV/US reactor design with increased flexibility and capability of responding effectively to variable water flow and quality conditions without the need for necessarily added chemicals or catalysts. Also, there is a need for a process to destroy halogenated organic pollutants using UV/US. This invention addresses these needs.
The present invention is a process and reactors for simultaneous ultraviolet light/ultrasound (UV/US) treatment of pollutants (e.g., halogenated organic compounds) in water. This process and these reactors use an advanced oxidation process (AOP) for the treatment of waters (e.g., surface water, groundwater). The process and reactors are designed to combine photolysis/photocatalysis and sonolysis, induced by ultraviolet light (UV) with/without photocatalyst (e.g., TiO2) and ultrasound (US), respectively. The process and reactors are preferably circular, cylindrical-shaped reaction vessels that accept a central ultrasonic horn. UV light is provided by lamps placed generally parallel to the reactor walls, either external to the walls when a material transparent to UV is used for the reactor walls, or centrally provided in a transparent immersion well. Between, for example, the UV immersion well near the top of the reactor, and the ultrasonic horn near the bottom of the reactor, a horizontal, hollow metal partition with reactant flow-through holes is installed. This way, simultaneous UV/US energy may be effectively provided to the reactors for the remediation of pollutants in the water in the reactors. Also, this way, compact and portable reactors may be constructed to permit mobile applications of the UV/US processes. Because the units may be made mobile, they can be used for military applications, for remote or temporary communities or for emergency responses, as after natural disasters or accidental contaminations of water supplies.